The present invention relates generally to wind turbines and, more particularly, to a wind turbine having blades which simultaneously, and under naturally occurring forces, fold straight downwind and feather to prevent damage to the turbine.
Wind turbines have been used for years in an attempt to capture the energy carried by moving air, with some turbines being designed for low speed, but high torque, operation, such as to operate pumps, and others operated at high speed and less torque, which are typically utilized to operate electric generators, or the like.
Wind turbines may be mounted on vertical or horizontal axes, with the horizontal axis turbines normally being designed so that the rotational axis lies in an up/down wind direction, and is freely rotatably mounted about a vertical axis to automatically maintain this orientation. Some horizontal axis turbines have the blades mounted on the upwind end of the rotational axis, while others have the blades mounted on the downwind end of the axis. The turbines having the blades on the upwind end normally utilize a simple tail vane or some other special equipment to keep the blades oriented into the wind. The downwind blade turbines normally utilize the wind forces themselves to maintain a correct orientation of the turbine relative to the wind.
The movement of a blade across the path of an airstream creates the effect of a relative wind blowing on the blade from the direction in which the blade travels. This relative wind, at least in high speed turbines, is normally much higher in velocity than the speed of the moving air mass through which the blade moves, and combines with the original wind velocity perpendicular to it, to result in an apparent wind on the blade.
Power which can be extracted from moving air varies with the cube of the velocity of the wind. For good efficiency, two-thirds of the wind velocity is converted to turbine rotation energy, and the remaining one third is left to continue transport of the air to avoid stagnation.
Pressures on the blade surface vary as the square of the wind speed. These pressures are higher on the upwind side of the blade, and lower on the downwind side. The resultant force typically acts through a point approximately one quarter of the distance back from the leading edge of the blade toward the trailing edge, and acts in a direction fairly perpendicular to the blade surface, largely in the downwind direction, but partially in the direction of rotation.
The resultant aerodynamic or wind force on a blade can be considered to be constituted of a lift force perpendicular to the apparent wind direction, and a smaller drag force along the apparent wind direction.
The resultant force can also be considered to be separated into a large force which acts in the direction of the main axis of the turbine, and a small force acting perpendicular to it in the direction the blade rotates. This latter force is a component which, with the movement of the blade, represents the converted energy of the turbine.
It can be seen from the above that energy capture potential of a turbine varies with the square of the diameter of interception area of the blade, with the cube of wind velocity, and, also, with air density, and humidity, which have not been mentioned. A well designed blade profile and pitch schedule are important to the efficiency of a wind turbine.
For high speed wind turbines, two, three, or four blades are normally used. Two blades are normally slightly more efficient, but produce problems with dynamics. Four blades are less efficient and more expensive, leaving a three blade configuration to be probably the best alternative for most mid-sized wind turbines.
Probably the biggest concern with high speed wind turbines is the survivability of the turbine, i.e., its ability to withstand high wind velocities and turbulence. In attempts to improve the survivability of a high speed wind turbine, mechanical arrangements have been made to turn and tilt the blades to keep them from rotating too fast. Many of these systems have been overly complex, and are detrimental not only from an economic standpoint, but, also, from a dependability standpoint. As will be appreciated, in order to improve survivability, the main objective is to limit rotor speed so as to avoid destructive levels of centrifugal forces and imbalance problems.
One system for improving survivability has been to build the system of very heavy and strong material, so that nothing can break at any speed. This philosophy is possibly alright for smaller machines, but cannot be economically employed on large machines, due to material bulk increasing faster than energy capture.
Another system for limiting rotor speed is to rotate the rotor assembly out of its normal position, bringing it sideways relative to the wind direction. If this is not done manually, or with a simple fantail spinner, then the complexity and cost of equipment to accomplish this rotation can be very large, with opportunity for malfunctioning increasing, and Exposure to cross winds which often occur in eddies in the strong winds.
Feathering the blades, i.e., rotating the blades about a longitudinal axis of the blade, has also been employed to limit rotor speed. While feathering is an effective protection, it normally has been done with blades that are left in their normally extended positions, perpendicular to their axis of rotation, where they develop large drag forces, which endanger the blades and the supporting tower.
Another procedure to limit rotor speed has been to fold the blades downwind as they are rotating. This is an excellent tactic for limiting the rotor speed, but the very large centrifugal forces normally acting on the blades tend to resist the folding of the blades, unless extraordinary force is applied to fold them, and due to conservation of momentum, the folding tends to increase rotating speeds.
It has also been proposed that the blades be feathered and then folded to control rotation speed, but the mechanical system for manipulating the blades in this manner is very complex, lending to excessive costs and dependability problems.
Brakes have also been used to slow turbines down in high wind conditions, but this approach may cause frequent power interruptions in normal operation.
To applicant's knowledge, no wind turbine has been developed which simultaneously folds the blades directly or straight downwind and feathers the blades to protect the turbine from high winds through natural forces and without use of auxiliary control equipment. It is to this end that the present invention has been developed.